Modified superhard cutting elements having reduced surface roughness method of modifying, drill bits equipped with such cutting elements, and methods of drilling therewith

ABSTRACT

A superhard cutting element having a polished, low friction substantially planar cutting face with a surface finish roughness of 10μ in. or less and preferably 0.5 μ in. or less. A chamfered cutting edge and side surface of the superhard material table of the same surface finish roughness are also disclosed. The surface roughness of the aforementioned superhard material table portions may be reduced by mechanically smoothing and polishing according to one of several variations of the method of the invention.

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/1156,086 filed Nov. 22, 1993, U.S. Pat. No. 5,447,208.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to cutting elements for use withrotary drag bits, and more specifically to superhard cutting elementshaving an extremely smooth finish on at least a portion of the leadingsurface or cutting face.

2. State of the Art

Superhard cutting elements have been commercially available for over twodecades. The typical superhard cutting element employed on rotary dragbits for earth boring comprises a substantially planar polycrystallinediamond compact (PDC) table supported by a metal substrate, typically ofsintered tungsten carbide, although other metals and combinations ofmetals have been employed over the years. The cutting face on the vastmajority of commercially available PDC cutting elements is a planar,circular surface. In state of the art PDC cutting elements, the cuttingface is typically lapped to a smooth finish.

In some subterranean formations, PDC cutting elements have been veryeffective in cutting the formation as the drag bit carrying the cuttingelements rotates and the edge of the cutting surface engages theformation. However, in certain formations which fail plastically, suchas highly pressured or deep shales, mudstones, siltstones and somelimestones, as well as other ductile formations, the formation cuttingshave a marked tendency to adhere to the leading surface or cutting faceof the cutting element.

When cuttings adhere to the cutting face of a cutting element, they tendto collect and to build up as a mass of cuttings adjacent to the pointor line of engagement between the cutting face of the PDC cuttingelement and the formation, potentially increasing the net effectivestress of the formation being cut. This build up of cuttings moves thecutting action away from and ahead of the edge of the PDC cuttingelement and alters the failure mechanism and location of the cuttingphenomenon so that cutting of the formation is actually effected by thisbuilt up mass, which obviously is quite dull. Thus, the efficiency ofthe cutting elements, and hence of the drag bit is drastically reduced.

Undesired adhesion of cuttings from subterranean formations beingdrilled by drag bit PDC cutting elements has long been recognized as aproblem in the subterranean drilling art. A number of differentapproaches have been attempted to facilitate removal of formationcuttings from the cutting face of PDC cutting elements. For example,U.S. Pat. No. 4,606,418 to Thompson discloses cutting elements having anaperture in the center thereof which feeds drilling fluid from theinterior of the drill bit onto the cutting face to cool the diamondtable and to remove formation cuttings. U.S. Pat. No. 4,852,671 toSouthland discloses a diamond cutting element which has a passageextending from the support structure of the cutting element to theextreme outermost portion of the cutting element, which is notched inthe area in which it engages the formation being cut so that drillingfluid from a plenum on the interior of the bit can be fed through thesupport structure and to the edge of the cutting element immediatelyadjacent the formation. U.S. Pat. No. 4,984,642 to Renard et at.discloses a cutting element having a ridged or grooved cutting face onthe diamond table to promote the break-up of cuttings of the formationbeing drilled or in the case of a machine tool, the material beingmachined, which enhances their removal from the cutting face. Theirregular topography of the cutting face itself assists in preventinghulling or clogging of the drag bit by reducing the effective surface orcontact area of the cutting faces on the cutting elements, which alsoreduces the pressure differential of the formation chips being cut. U.S.Pat. No. 5,172,778 to Tibbitts et al., assigned to the assignee of thepresent application, employs ridged, grooved, stair-step, scalloped,waved and other alternative non-planar cutting surface topographies topermit and promote the access of fluid in the borehole to the area onthe cutting element cutting face immediately adjacent to and above thepoint of engagement with the formation, to equalize differentialpressure across the formation chip being cut by the cutting element andthus reduce the shear force which opposes chip movement across thecutting surface. U.S. Pat. No. 4,883,132 to Tibbitts, assigned to theassignee of the present application, and incorporated herein byreference, discloses a novel drill bit design providing large cavitiesbetween the face of the bit and the cutting elements engaging theformation, so that formation cuttings enter the cavity area where theyare unsupported and thus more likely to break off for transport up theborehole. Clearing of the cut chips is facilitated by nozzles aimed frombehind the cutting elements (taken in the direction of bit rotation) sothat the chips are impacted in a forward direction to break offimmediately after being cut from the formation. U.S. Pat. No. 4,913,244to Trujillo, assigned to the assignee of the present invention,discloses bits which employ large cutters having associated therewithdirected jets of drilling fluid emanating from specifically orientednozzles placed in the face of the bit in front of the cutting elements.The jet of drilling fluid is oriented so that the jet impacts betweenthe cutting face of the cutting element and a formation chip as it ismoving along the cutting face to peel it away from the cutting elementand toward the gage of the bit. U.S. Pat. No. 4,976,324 to Tibbitts,assigned to the assignee of the present invention, disclosesdiamond-film coated diamond cutting elements of various types such aspolycrystalline diamond compact, thermally stable polycrystallinediamond compact (commonly termed TSP's or thermally stable products),Mosaic® cutting elements formed of an array of TSP's, natural diamondsand diamond impregnated cutting elements. The diamond film has a lowerporosity or higher purity than the underlying diamond substrate, thusimproving the quality, uniformity, and sharpness of the cutting edge ofthe cutting element, assisting in resisting wear of the cutting elementand improving impact resistance of the cutting surface. U.S. Pat. No.5,115,873 to Pastusek, assigned to the assignee of the presentapplication, discloses yet another manner in which formation cuttingscan be removed from a cutting element by use of a structure adjacentand/or incorporated with the face of the cutting element to directdrilling fluid to the face of the cutting element behind the formationchip as it comes off the formation. U.S. Pat. No. 4,988,421 to Drawl etat. discloses a method of toughening the structure of a diamond ordiamond-like, coated tool by depositing by low pressure chemical vapordeposition several layers of diamond or diamond like particles onto anon-diamond or non-diamond-like tool substrate.

None of the foregoing approaches to cutter and bit design have beencompletely successful in practice in achieving the desired result offacilitating chip removal from the face of the cutting element.Moreover, it will be appreciated by those skilled in the art that all ofthe foregoing approaches require significant modification to the cuttingelements themselves, to the structure carrying the cutting elements onthe bit face, and/or to the bit itself. Thus, all of the foregoingapproaches to the problem require significant expenditures andsubstantially raise the price of the drill bit. In addition, due torequired cutter placement on certain styles and sizes of bits, many ofthe prior art hydraulic chip removal arrangements are unsuitable forgeneral application. Accordingly, it would be extremely desirable toprovide the industry with a solution to the impairment to the cuttingmechanism caused by chip adhesion, which solution could be economicallyeffected on any drill bit regardless of size or style, and regardless ofthe type of formation which might be expected to be encountered by thedrill bit.

SUMMARY OF THE INVENTION

The present invention provides a cutting element including a table ofsuperhard material having a leading surface or curing face of low orreduced surface roughness in comparison to prior art cutting elements.While the present invention's primary applicability is topolycrystalline diamond compact (PDC) cutting elements havingsubstantially planar diamond tables, the term "substantially planar" isintended and understood to include concave, convex, and other nonlineardiamond table surfaces which nonetheless employ a substantial cuttingsurface primarily in two dimensions and of any surface shape, topographyor geometry, to engage and cut the formation. The term "Substantiallyplanar" is also intended and understood to encompass cutting elementshaving grooved, ridged or other non-planar interfaces between thediamond table and the supporting WC substrate.

The superhard cutting element according to the present invention is alsointended and understood to include variations of polycrystalline diamondcompact and other superhard cutting structures. For example, the term"superhard" includes without limitation the previously referencedthermally stable products or TSP's, Mosaic® cutting elements formed fromTSP's disposed in a planar army, diamond films and cubic boron nitridecompacts as well as other superhard materials known in the art.

The present invention comprises a superhard cutting element having asubstantially planar cutting table providing a surface having a cuttingedge for engaging a formation to be drilled by the rotary drag bit uponwhich the cutting element is mounted. The aforementioned surface, whichis generally oriented to comprise the leading surface or cutting face ofthe cutting element, taken in the direction of bit rotation, isprocessed to a smoothness far in excess of that normally provided instate of the art, commercially available cutting elements.

In the most preferred embodiment of the present invention, the leadingsurface or cutting face of the cutting element is mechanically,chemically or otherwise polished substantially to a "mirror" finish inwhich one can literally see reflections of objects. Reduced perturbationof the polished curing face lowers the coefficient of friction of thecutting face, and greatly reduces adhesion of the formation chips. Inaddition, such polishing reduces potential fracture nucleation sites orflaw sites in the cutting face.

The significant reduction in adhesion reduces the shear stress orresistance to movement of chips on the cutting face, and thus the normalforces as well as tangential forces required for a specified depth ofcut in a specific formation. A reduction in normal forces in real-worldterms translates to a reduction in the drill string weight required tobe applied to the drill bit down hole or an increase in depth of cut andrate of penetration for a given weight on bit, while a reduction intangential forces translates in real-world terms to a reduction in thetorque required to rotate the bit for a given depth of cut and rate ofpenetration. The reduction of these cutting forces translates toimproved drilling efficiency, and improved durability and longevity ofthe drill bit.

It is also demonstrated that formation chips from formations which failplastically or which behave in a ductile manner, which are cut by apolished, substantially mirror-finish, superhard cutting element cuttingsurface under certain drilling conditions, are generally ribbon-likewith a substantially uniform thickness and are removed from theformation being drilled in a continuous fashion. This phenomenon is incontrast to the chips cut by a standard superhard cutting element withan unpolished surface under similar conditions, such chips beingglobular with irregular configuration and building up in a packed masson a cutting face of the cutting element to a thickness two to threetimes the thickness of those on the polished cutting face before movingup the cutting face to the bit face and releasing to be removed up hole.Moreover, the polished cutting elements of the present invention achievea freely machined "kerf" or bottom hole pattern, and thus a smootherborehole bottom, lowering bit vibration and chattering and maintainingmore continuous contact between the bit face and the bottom of theborehole.

It has also recently been discovered by the inventors that the cuttingelement of the present invention provides advantages in reduction ofcutting energies and increased ROP in non-plastic formations, such as,for example and not by way of limitation, sandstones, limestones andshales. While the exact mechanism of these advantages is not fullyunderstood, laboratory testing of cutting element performance hasconfirmed its existence.

The invention also contemplates the smoothing and polishing of otherportions of a cutting element, such as, for example, one or morechamfers located along at least a portion of the cutting edge of thetable of superhard material, as well as the side of the table to therear of the chamfer. Reduced surface roughness cutting element mountingstructures such as studs and bit bodies having selected reduced surfaceroughness areas such as waterways and junk slots are also includedwithin the ambit of the invention.

The invention further contemplates a method of reducing surfaceroughness of the cutting faces and other areas, such as theaforementioned chamfers and sides, of the superhard material tables ofcutting elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one type of typical superhard cuttingelement having a substantially planar cutting face;

FIG. 2 is a perspective view of a rotary drag bit having PDC cuttingelements mounted thereon;

FIG. 3 is a side elevation of a prior art superhard cutting element asit engages and cuts a subterranean formation, depicting the manner inwhich formation chips cut from the formation can build up ahead of thecutting face and impede the cutting process and removal of chips fromthe cutting element;

FIG. 4. is a side elevation of a superhard cutting element according tothe present invention having a polished or mirror finish, depicting thecontinuous and uniform manner in which a formation chip is cut andremoved from the formation without build-up ahead of the cutting face;

FIG. 5A is an exemplary rendering of a side sectional elevationillustrating the topography of a prior art superhard cutting elementcutting face;

FIG. 5B is an exemplary rendering of a side sectional elevationillustrating the topography of a superhard cutting element cutting faceaccording to the present invention;

FIGS. 6A-6C are front elevations depicting cutting elements havingpolished chamfers and sides and partially polished cutting faces, allaccording to the present invention; and

FIG. 7 is an enlarged side elevation depicting a cutting elementincluding a chamfered cutting edge and a tapered side surface extendingfrom the chamfer along the side of the superhard material table andsubstrate supporting the table, at least a portion of the cutting face,chamfer and table and substrate sides being polished.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1 of the drawings, an exemplary superhard cuttingelement 10 is depicted in perspective view. Cutting element 10, in thisinstance a PDC, comprises a diamond table 12 supported on an underlyingsubstrate 14, which is typically of sintered tungsten carbide particlesor of any other suitable composition. The boundary between diamond table12 and substrate 14 may be planar as shown, or may be irregular asillustrated in U.S. Pat. Nos. 4,784,023, 5,011,515, 5,120,327 and5,351,772, the last of which is assigned to the assignee of the presentapplication and incorporated herein by this reference. The nature orconfiguration of the boundary between the diamond table 12 and substrate14 does not, however, form a part of the present invention and so willnot be further described.

In use, as mounted on the rotary drag bit, the superhard cutting element10 is generally supported by an additional carrier structure such as acylinder or, as shown in FIG. 1, a so called "stud" 16 having a carriersurface 18 to which the rear surface 20 of substrate 14 is bonded, as bybrazing. Stud 16 also includes a free end 22, which is inserted andsecured in an aperture drilled or otherwise formed in the face of therotary drag bit to which the superhard cutting element 10 is to bemounted. Alternatively, the carder structure 16 may comprise acylindrical substrate supporting a diamond table, which substrate isbrazed into a socket or pocket formed in the face of the rotary drag bitwhen the bit is fabricated and furnaced, or formed thereafter bymilling, drilling or other machining process. Both studs and cylindersare typically formed of materials compatible with that of substrate 14,such as the aforementioned tungsten carbide.

FIG. 2 of the drawings depicts a drill bit 30 having a body 32 securedto a shank 34 having a threaded pin connection 36 thereon, whereby bit30 is made up to the end of a drill string or to a down hole motordisposed to the end of a drill string. PDC cutting elements 10 are shownsecured in a predetermined pattern and at predetermined heights andorientations on the face of the drill bit 30 to provide effectivecutting for the formation type to be cut, nozzles 38 on body 32 beingpositioned to clear chips of formation material leaving cutting elements10.

Returning to FIG. 1, the diamond table 12 of PDC cutting element 10possesses a leading surface face or cutting face 40, the outermost edge42 of which (as the cutting element 10 is mounted to the body 32 ofdrill bit 30) may be defined as the cutting edge 42 by which the cuttingelement 10 engages and cuts the formation. In PDC cutting elements ofthe prior art, cutting face or leading face 40 of PDC cutting element 10would typically be lapped to a surface finish of 20-40μ in. RMS (allsurface finishes referenced herein being RMS) which is relatively smoothto the touch and visually planar (if the cutting face is itself flat),but which includes a number of surface anomalies and exhibits a degreeof roughness which is readily visible to one even under very low powermagnification, such as a 10x jeweler's loupe.

Referring now to FIG. 3 of the drawings, a cutting element 10 is shownmounted on body 32 of a rotary drag bit 30 as bit 30 is inverted fromthe position shown in FIG. 2 and so oriented for drilling in asubterranean formation. Formation 50, which by way of example may be anaforementioned shale, such as a Catoosa shale, is being engaged by PDCcutting element 10, it being readily seen that the curing edge 42 as isimpeded from engaging the pristine or completely uncut portion 52 offormation 50 by virtue of the large build-up of formation cuttings 54ahead of cutting face 40 and under cutting edge 42. The irregularformation chip 56, which ultimately extends from the build-up 54 on theleading face 40 of cutting element 10, is actually more or less extrudedfrom the massive build-up of formation chips riding against the face 40of PDC cutting element 10, and not cut directly from the formation 50,so failure of the formation material occurs at area 58.

It is thus readily apparent that this undesirable build-up of formationmaterial in advance of the PDC cutting elements mounted across the faceof the rotary drag bit impairs the cutting action of the PDC cuttingelement as the cutting process can actually be effected by this built-upmass of cuttings moving ahead of the actual cutting element itself. Asone might imagine, the formation chips are quite dull and cutting iseffected in a manner which may be analogized to pushing a dull plowthrough damp earth, whereby earth is moved out of the way by the merephysical presence, mass and movement of the plow blade surface with anadhered compacted coating of earth, rather than the dirt clods beingsevered from the underlying earth by a sharp-edged plow blade. Thus, thenormal force, or in real terms the weight on bit, which needs to beapplied to the bit to effect a desired depth of cut and rate ofpenetration through the formation must be made undesirably and, in somecases, unreasonably high. In a similar manner the tangential forces orthe torque required to rotate the bit at the bottom of the borehole insuch a situation is again undesirably increased, as the superhardcutting elements are merely moving the formation chips out of the way bysheer force, being unassisted by the relatively sharp edge 42 ofsuperhard cutting element 10. Stated another way, the required normaland tangential forces are both increased due to the large bearing areaprovided by the build-up of formation material at the cutting edge ofthe cutting element. The net result is an extremely inefficient rockcutting removal mode, which in some circumstances and in certainformations may actually cause a cessation of drilling.

Referring now to FIG. 4 of the drawings, a cutting element 10' similarto cutting element 10 is depicted engaging and cutting the samesubterranean formation 50. The substantial difference between the twocutting elements is that the cutting face 40 of the superhard cuttingelement 10' has been physically modified, as by polishing to a mirrorfinish of 0.5μ in. As illustrated, it will readily be seen that thecutting edge 42 of cutting element 10' is fully engaged with thepristine or previously uncut and Undisturbed area 52 of subterraneanformation 50, failure of the formation material occurring immediatelyadjacent cutting edge 42. Thus, cutting edge 42 is able to cut or sheara formation chip 156 from the formation in an unimpeded manner. Asshown, formation chip 156 of substantially uniform thickness movesrelatively freely from the point of contact or line of contact fromcutting edge 42 of cutting face 40 upwardly along the cutting face 40until it breaks off either by contact with the body 32 of bit 30 or dueto impact by drilling fluid emanating from a nozzle 38 on the face ofthe bit body 32, or fluid coursing through a channel on the face of thedrill bit. Alternatively, a so-called "chip breaker" as known in the artmay be utilized directly above and adjacent cutting face 40 of superhardcutting element 10' for chip 156 to contact and fracture at the point ofcontact for removal by the drilling fluid to the surface. The polishedor mirror finish provided on cutting face 40 of superhard cuttingelement 10' lowers the overall stresses applied to the rock in thecutting area and permits the chip 156 to ride smoothly due to reducedsliding friction in an unimpeded manner up the cutting face.

Referring to FIGS. 5A and 5B of the drawings, the difference in surfacetopography between the cutting face 40 of a prior art PDC cuttingelement 10 and that of a PDC cutting element 10' according to thepresent invention will be readily appreciated. FIGS. 5A and 5B compriserenderings as if a diamond or other superhard table were sectionedperpendicular to the cutting face, and not tracings of actualphotomicrographs. In FIG. 5A, cutting face 40 of superhard table 12 ofprior art cutting element 10 is shown to contain microscopic "peaks" 100and "valleys" 102 in the surface 104. Such minute elements may always bepresent, as well as large "ranges" or "waves" 106 and "canyons" or"troughs" 108 which comprise the major topographic features orperturbations on cutting face 40. It is these ranges or waves 105 andcanyons or troughs 108 and not the much smaller microscopic peaks 100and valleys 102 in surface 104 which provide or result in the 20-40μ in.surface roughness of the cutting face 40 of prior art cutting element10. FIG. 5B, on the other hand, depicts how such waves or ranges 106 areof markedly reduced height and canyons or troughs 108 of markedlyreduced depth in the cutting element 10' of the present invention.Broken line 110 provides a reference baseline within each table 12 fromwhich to view the relative surface roughness of cutting element 10 and10'. Thus, in microscopic terms, the surface smoothing which takes placein producing a cutting element in accordance with the present inventioneffects a modification and reduction of relatively large-scale featuresof the surface topography, and not an elimination of individualinclusions in and protrusions from the surface itself. 0f course, somesignificant reduction in potential nucleation sites or flaw sites isachieved, as previously noted.

It might be expected in the case of the polished or mirror finishedcutting face of superhard cutting element 10' that the differentialpressure on the outer or leading side 60 of the formation chip, the sideaway from cutting face 40, and that on the inner side 62 on theformation chip immediately abutting the curing face 40, would cause chip156 to strongly stick or adhere to the cutting face 40 due to thepressure differential. However, drilling laboratory tests have clearlyshown that this is not the case, and that the smoothness and attendantreduced coefficient of friction of cutting face 40 substantially reducesthe effect of the phenomenon of differential pressure sticking. Fieldtests of bits having polished cutting face PDC cutting elements havealso confirmed the apparent dominance of the reduction of thecoefficient of friction over the increase in differential pressureattributable to the smoothness of the cutting face.

Single point or single cutter cutting tests in drilling simulators havebeen performed with PDC cutting elements having standard lapped finishand those which have been polished to a substantially mirror finish. Incutting tests employing General Electric Compax® cutters 9.2 mm square,backraked at an angle of 15°, performed on samples of Catoosa shaleunder a simulated down hole pressure of 9000 psi with a 0.005 inch depthof cut and a rotational speed of 273 rpm, the mirror finish cuttingelements significantly outperformed the standard lapped finish cuttingelements in terms of the force required to achieve a specified depth ofcut and rate of penetration (ROP).

For example, in tests performed with these sharp, unchamfered PDCcutting elements, reduction in normal forces (weight on bit) ofapproximately 60% and reduction in tangential forces (torque) of 35% forthe cutter with the polished diamond table were achieved. A second setof comparative single point cutting tests were run with polished andlapped cutting elements having a 0.010 inch by 45° chamfer ground on thecutting edge on each cutting element. These tests produced similarresults in proportion to those of the first tests with the sharp,unchamfered edges, slight increases in the loads required to cut theformation being experienced due to the chamfered cutting edges of thecuring elements.

In addition to the relative reduction in normal and tangential loadingexperienced using polished cutting faces, there was also a markeddifference in the appearance of the formation chips and kerf (troughleft by the cutting element) produced during the cutting tests. Thechips cut by the polished cutting face PDC cutting element were ofsubstantially uniform thickness and substantially continuous appearance.In comparison, the formation chips cut by the standard or unpolishedcutting element appear to have convoluted and built up on the cuttingface of the cutting element (see FIG. 3) to a thickness two to threetimes the thickness of those cut by the polished cutting element beforebreaking loose from the built-up mass and then sliding up the face ofthe cutter. The kerf cut by the polished cutting element was verysmooth, almost machined in uniformity, while the kerf cut by thestandard lapped cutting element possessed an irregular profile andbottom surface.

More recently, tests of cutting elements according to the presentinvention in comparison to standard lapped finish cutting elements withnon-plastic rock, specifically sandstone, have revealed a 100 percentincrease in ROP. Thus, the polished cutting element of the presentinvention is believed to possess advantages when drilling sandstones,limestones and shales as well as the previously-referenced plastic orductile formations.

By way of example and not limitation, one mode currently known to theinventors for polishing the cutting face 40 of superhard cuttingelements 10 to obtain cutting elements 10' of the present invention islapping of the cutting face on conventional cast iron laps known in theart using progressively smaller diamond grit suspended in a glycol,glycerine or other suitable carrier liquid. The lapping is conducted asa three step process commencing with a 70 micron grit, progressing to a40 micron grit and then to a grit of about 1-3 microns in size. Incontrast, standard lapping techniques for a prior art PDC cuttingelement 10, which may follow an initial electrodischarge grinding of thecutting face, finish lap in one step with 70 micron grit. By way ofcomparison of grit size, 70 micron grit is of the consistency of freesand or crystalline material, while 1-3 micron grit is similar inconsistency to powdered sugar.

However, it has also been established by the inventors that the normal20-40μ in. surface roughness, averaging 30μ in., of state-of-the-art PDCcutting elements may be smoothed to the finish of the invention in aone-step process. The cutting elements, as received from themanufacturer, are placed with their cutting faces against a dry,rotating diamond wheel. A preferred wheel is the Winter RB778 resinbonded diamond wheel, offered by Ernst Winter & Sons, Inc. of TravelersRest, S.C. It is important that the wheel be cooled as the diamond wheelis of resin bonded construction. Elevated temperatures would result inthe destruction of the wheel. The nature of the polishing processrequires that the abrasive surface be kept dry. However, the wheel maybe moistened with water at the start of the polishing process to reducedrag and facilitate proper orientation of the PDC cutting faces againstthe wheel. In addition, the recommended temperature range, whereinpolishing can be effected, is from 140° F.-220° F. While the polishersemployed by the inventors rotate at 3500 RPM, it is believed that arange of from 3000-5000 RPM would likely be adequate. About 2 lb. toabout 8 lb. force is applied to the cutting elements against the wheel.As noted below, it is preferred that the finish of the cutting faces besmoothed to a 0.5μ in. or less surface finish roughness approaching atrue "mirror" finish. It takes about fifty minutes to an hour ofpolishing with the aforementioned diamond wheel to achieve this finishwith a state-of-the-art nominal one-half inch diameter PDC cuttingelement cutting face, and one and one-half to two hours for a nominalthree-quarter inch diameter PDC cutting face. The same methodology forpolishing cutting faces may be applied to polish a chamfer at thecutting edge of the cutting face, as well as the side of the superhardtable to the rear of the chamfer. To polish such surfaces, the cuttingelements, held by their substrates, are disposed at the desired angle tothe rotating wheel. The cutting elements are themselves rotated abouttheir axis of symmetry to smooth and polish the desired chamfer and sideareas of the superhard table.

In developing the above-described single-step smoothing and polishingtechnique, the inventors have come to appreciate that such techniqueappears to desirably reduce both the aforementioned ranges or waves 106and canyons or troughs 108 in magnitude as well as the perceivedincidence and magnitude of smaller asperities such as peaks 100 andvalleys 102 (see FIGS. 5A and 5B). While state-of-the art profilometersmay be hard-pressed to quantitatively measure reduction of such "small"asperities, reflectivity measuring techniques developed by the inventorsusing laser light reflected from cutting faces of PDC cutting elementsto a laser power meter have indicated that the quality of the preferredmirror finish cutting surface of the invention is governed to a certainextent by surface structures of different relative scales. As aconsequence, it appears that reduction of the incidence and magnitude of"small" asperities in combination with removal and reduction of thelarger asperities may further enhance finish quality. It should beappreciated however, that the terms "large" and "small" as applied tosuch surface asperities are relative and used for purposes of mutualcomparison only, all of such asperities being minute in the absolutesense relative to the dimensions of the cutting face, chamfer or othersurface in question. Thus, as previously and subsequently mentionedherein, one could smooth and polish a curved, ridged, waved or othercutting face in accordance with the present invention to remove andreduce both large and small asperities, resulting in a mirror finishcutting face which nonetheless is not flat in the absolute sense.

It is also contemplated that the cutting element cutting surfaces(cutting face, chafer, side, etc.) may be polished by other means, suchas ion beams or chemicals, although the inherently inert chemical natureof diamond makes the latter approach somewhat difficult for diamond.

While an industry-standard PDC or other superhard cutting elementtypically has a lapped surface finish on the cutting face withirregularities or roughness (measured vertically from the surface) onthe order of 20-40μ in., as a result of the above-described polishing,the most preferred embodiment of the superhard cutting element of thepresent invention possesses a cutting face surface finish of about 0.3to 0.5μ in. or about 0.4 to 0.6μ in. roughness. It appears that asuperhard cutting element surface finish with a substantially 10μ in.roughness provides some of the benefits previously described, although asubstantially 2μ in. or less surface finish roughness is more preferred,and a substantially 0.5μ in. or less surface finish roughnessapproaching a true "mirror" finish is most preferred. The foregoingdimensional irregularities on the cutting face are as measured using acalibrated Hommel America Model T-4000 diamond stylus profilometercontacting the cutting surface of the cutting element.

To quantify the results achievable by the present invention, when a PDCcutting element is polished to 0.5μ in. mirror finish, calculationsbased upon force data show the coefficient of friction to be reduced toabout half, or fifty percent, of that of a 20-40μ in. standard finished,but otherwise identical, PDC cutting element. Thus, it can be said thatreducing sliding contact stresses between the curing face and formationchip can be defined macroscopically as achieving a low friction PDC,diamond or other superhard material table.

It has been proposed in the art (see U.S. Pat. No. 4,976,324) that adiamond film provides a smooth, low friction finish, either alone or asdeposited on an underlying substrate, such as a PDC diamond table.However, the inventors have come to recognize that such is notnecessarily the case, and that significant reductions in the coefficientof friction of even diamond film surfaces may be achieved by polishingas previously described. This is somewhat unexpected and surprising,since diamond already inherently has one of the lowest knowncoefficients of friction of any material. While in some cases thereduction in friction achieved by polishing a diamond film may not be asphenomenal as that noted above with respect to polished versus lappedPDC cutting elements, it is nonetheless substantial and demonstrable.

Another observed benefit of polishing a superhard cutting elementcutting face to a surface finish in the above-referenced beneficialrange of 10μ in. surface finish or finer is the marked reduction indegradation of the cutting face in use. Unlike temporary surfacecoatings or coverings used in the prior art (see U.S. Pat. Nos.5,135,061 and 5,025,874) to protect diamond tables while tripping a bitinto the borehole and during the first few minutes of drilling, thepolished superhard cutting face of the present invention retains itsrobust characteristics after many hundreds of feet of drilling.

While the present invention has been described with reference toindividual cutting elements mounted at separate locations on a bit face,it is contemplated that the invention has equal utility with blade-typebits wherein very large curing faces are presented to engage theformation such as those described in the aforementioned U.S. Pat. No.4,883,132 to Tibbitts and U.S. Pat. No. 4,913,247 to Jones, assigned tothe assignee of the present invention and incorporated herein byreference. Such cutting faces may be formed as Mosaic® cutters, or maybe fabricated from adjacent round, square or otherwise shaped individualcutting elements of the same or different material, closely spaced andwith cooperative or even interlocking borders. As noted previously,convex, concave or other arcuately-surfaced cutting elements may bepolished, as may the alternate geometry (stepped, ridged, waved, etc.)cutting element surfaces described in some of the previously-referencedprior art.

It is also contemplated that sides and chamfers of superhard materialtables of cutting elements may be processed to enhanced smoothnessaccording to the present invention, and that the benefits observed frompolished cutting faces will be further amplified. FIGS. 6A-6C of thedrawings depict cutting elements 10' according to the present inventionin which the chamfer 24 and side surface 26 of the diamond table havebeen polished, at least in the portion of the cutting element peripherywhich will engage the formation. FIGS. 6A-6C also illustrate anembodiment of the invention wherein only a segment or portion 140 ofcutting face 40 has been smoothed or polished. When only a portion ofthe cutting face, side or chamfer is polished, polishing time may bereduced and the unpolished portion of the cutting face may act as a chipbreaker when the formation chip encounters the unpolished area ofsubstantially greater frictional coefficient.

FIG. 7 of the drawings depicts a more specific structural embodiment ofthe present invention by illustrating a cutting element 210' of a designdeveloped by the assignee of the present invention and disclosed andclaimed in U.S. patent application Ser. No. 08/039,858, now U.S. Pat.No. 5,460,233 assigned to the present assignee and incorporated hereinby reference. Cutting element 210' includes a cutting face 240 boundedby a chamfer 224 at the cutting edge 242, and a tapered or frustoconicalside surface 226 on the diamond (PDC) table 212, the taper of the sidesurface 226 of table 212 extending along the exterior side surface ofthe substrate 214 which supports the diamond table 212. At least aportion of the cutting face 240, chamfer 224 and side surface 226 ofcutting element 210' (those portions actively engaged with the formationto cut same) are polished in accordance with the present invention.

In wear testing of cutting elements of the general configuration element210', it has been observed by the inventors that the smoother thesurface finish of the chamfer and of the tapered side surface, the moredurable the cutter, as determined by onset of cutter wear. Recentexperiments on a rock planer that were designed to evaluate the relativedurability of PDC cutting elements suggest the durability is sensitiveto the surface finish of the area of the diamond table (includingchamfer and side) actually contacting the rock. These observations weremade using cutting elements that had variations in surface finishresulting from slight variations in the finishing process, and not ofcutting elements having polished chamfers or sides per se. However, itappears that an improved surface finish which might be achieved bypolishing would reduce microfracture and abrasive wear of the diamondtable, abrasive wear being related to microfracture, while the converseis not necessarily true. While the surface of the diamond table does notform a "skin," nonetheless a smooth diamond table surface may beanalogized to a protective skin which inhibits deterioration of thediamond table during the cutting process. Therefore, it is specificallycontemplated that the polishing of the chamfer 224 and tapered sidesurface 226 of such cutting elements be effected when the cutting face240 is polished. Preferred surface finish roughnesses for polishedchamfer and side surface portions have yet to be conclusivelyestablished, but are currently believed to coincide with thosepreviously disclosed with respect to cutting faces.

It is further contemplated that cutting element mounting structures andhigh erosion-and abrasion-susceptible areas on the bit body, such aswaterways 70 and junk slots 72 as illustrated in FIG. 2 of the drawings,may also benefit in terms of integrity and longevity from polishing toan extremely free surface finish.

While the present invention has been described in terms of certainpreferred embodiments, it is not so limited, and those of ordinary skillin the art will readily recognize and appreciate that many additions,deletions and modifications to the embodiments described herein may bemade without departing from the scope of the invention as hereinafterclaimed.

What is claimed is:
 1. A method for cutting a rock formation undersubterranean wellbore pressure, comprising:providing a mass of superhardmaterial defining a cutting face oriented substantially transverse to anintended direction of travel thereof and a cutting edge at a peripheryof said cutting face; engaging said rock formation under saidsubterranean wellbore pressure with said cutting edge and moving saidcutting face in said direction of travel; cutting a chip of materialfrom said rock formation with said cutting edge, said chip passing overand contacting at least a portion of said cutting face with a surface ofsaid chip overlying said contacted cutting face portion exposed to saidsubterranean wellbore pressure such that a pressure differential tendsto adhere said chip to said cutting face portion and impede movement ofsaid chip thereover; and substantially overcoming said differentialpressure-induced adherence tendency by providing said cutting faceportion with a polished surface finish exhibiting a sufficiently lowcoefficient of friction.
 2. A method for cutting a rock formation undersubterranean wellbore pressure, comprising:providing a mass of superhardmaterial defining a cutting face oriented substantially transverse to anintended direction of travel thereof and a cutting edge at a peripheryof said cutting face; engaging said rock formation under saidsubterranean wellbore pressure with said cutting edge and moving saidcutting face in said direction of travel; cutting a chip of materialfrom said rock formation with said cutting edge, said chip passing overand contacting at least a portion of said cutting face with a surface ofsaid chip overlying said contacted cutting face portion exposed to saidsubterranean wellbore pressure such that a pressure differential tendsto adhere said chip to said cutting face portion and impede movement ofsaid chip thereover; and providing said cutting face portion with apolished surface finish of sufficient smoothness to substantiallyovercome said differential pressure-induced adherence tendency.
 3. Amethod for cutting a rock formation exhibiting plastic failureproperties under subterranean wellbore pressure, comprising:providing amass of superhard material defining a cutting face orientedsubstantially transverse to art intended direction of travel thereof anda cuing edge at a periphery of said cutting face; engaging said rockformation under said subterranean wellbore pressure with said cuttingedge and moving said cutting face in said direction of travel; cutting achip of material from said rock formation with said cutting edge, saidchip passing over and contacting at least a portion of said cutting facewith a surface of said chip overlying said contacted cutting faceportion exposed to said subterranean wellbore pressure such that apressure differential tends to adhere said chip to said cutting faceportion and impede movement of said chip thereover; and substantiallyovercoming said differential pressure-induced adherence tendency byproviding said cutting face portion with a polished surface finishexhibiting a sufficiently low coefficient of friction.
 4. A drill bitfor cutting a rock formation under subterranean wellbore pressure,comprising:a rotary drill bit body having a connector at one end thereoffor connection of said apparatus to a drill string component; and atleast one cutting element mounted to said bit body and including a massof superhard material defining a cutting face oriented substantiallytransverse to a direction of intended rotation of said bit body and acutting edge at a periphery of said cutting face whereby, uponengagement of said rock formation under said subterranean wellborepressure by said cutting edge of said at least one cutting element androtation of said bit, a chip of material cut from said rock formationwith said cutting edge passes over and contacts at least a portion ofsaid cutting face, with a surface of said chip overlying said contactedcutting face portion exposed to subterranean wellbore pressure such thata pressure differential tends to adhere said chip to said contactedcutting face portion and impede movement of said chip thereover, saidcutting face portion having a polished surface finish exhibiting asufficiently low coefficient of friction to substantially overcome saiddifferential pressure-induced chip adherence tendency and facilitatesubstantially unimpeded movement of said chip thereover.
 5. A drill bitfor cutting a rock formation under subterranean wellbore pressure,comprising:a rotary drill bit body having a connector at one end thereoffor connection of said apparatus to a drill string component; and atleast one cutting element mounted to said bit body and including a massof superhard material defining a cutting face oriented substantiallytransverse to a direction of intended rotation of said bit body and acutting edge at a periphery of said cutting face whereby, uponengagement of said rock formation under said subterranean wellborepressure by said cutting edge of said at least one cutting element androtation of said bit, a chip of material cut from said rock formationwith said cutting edge passes over and contacts at least a portion ofsaid cutting face, with a surface of said chip overlying said contactedcutting face portion exposed to subterranean wellbore pressure such thata pressure differential tends to adhere said chip to said contactedcutting face portion and impede movement of said chip thereover, saidcutting face portion having a polished surface finish of sufficientsmoothness to substantially overcome said differential pressure-inducedchip adherence tendency and facilitate substantially unimpeded movementof said chip thereover.
 6. A drill bit for cutting a rock formationexhibiting plastic failure properties under subterranean wellborepressure, comprising:a rotary drill bit body having a connector at oneend thereof for connection to a drill string component; and at least onecutting element mounted to said bit body and including a mass ofsuperhard material defining a cutting face oriented substantiallytransverse to a direction of intended rotation of said bit body and acutting edge at a periphery of said cutting face whereby, uponengagement of said rock formation under said subterranean wellborepressure by said cutting edge of said at least one cutting element androtation of said bit, a chip of material cut from said rock formationwith said cutting edge passes over and contacts at least a portion ofsaid cutting face, with a surface of said chip overlying said contactedcutting face portion exposed to subterranean wellbore pressure such thata pressure differential tends to adhere said chip to said contactedcutting face portion and impede movement of said chip thereover, saidcutting face portion having a polished surface finish exhibiting asufficiently low coefficient of friction to substantially overcome saiddifferential pressure-induced chip adherence tendency and facilitatesubstantially unimpeded movement of said chip thereover.
 7. A method forcutting a subterranean rock formation under subterranean wellborepressure, comprising:providing a mass of superhard material defining acutting face oriented substantially transverse to an intended directionof travel thereof and a cutting edge at a periphery of said cuttingface; engaging said subterranean rock formation with said cutting edgeand moving said cutting face in said direction of travel; cutting a chipof material from an uncut portion of said subterranean rock formationwith said cutting edge, said chip passing over at least a portion ofsaid cutting face in contact therewith; and preventing substantialadherence of said chip to said cutting face portion by providing saidcutting face portion with a polished surface finish exhibiting asufficiently low coefficient of friction to avoid a substantial build-upof material cut from said subterranean rock formation ahead of saidcutting face.
 8. A method for cutting a subterranean rock formationunder subterranean wellbore pressure, comprising:providing a mass ofsuperhard material defining a cutting face oriented substantiallytransverse to an intended direction of travel thereof and a cutting edgeat a periphery of said cutting face; engaging said subterranean rockformation with said cutting edge and moving said cutting face in saiddirection of travel; cutting a chip of material from an uncut portion ofsaid subterranean rock formation with said cutting edge, said chippassing over at least a portion of said cutting face in contacttherewith; and preventing substantial adherence of said chip to saidcutting face portion by providing said cutting face portion with apolished surface finish exhibiting a sufficiently low coefficient offriction to facilitate failure of said uncut subterranean formationimmediately adjacent said cutting edge.
 9. A method for cutting asubterranean rock formation under subterranean wellbore pressure,comprising:providing a mass of superhard material defining a cuttingface oriented substantially transverse to an intended direction oftravel thereof and a cutting edge at a periphery of said cutting face;engaging said subterranean rock formation with said cutting edge andmoving said cutting face in said direction of travel; cutting a chip ofmaterial from an uncut portion of said subterranean rock formation withsaid cutting edge, said chip passing over at least a portion of saidcutting face in contact therewith; and preventing substantial adherenceof said chip to said cutting face portion by providing said cuing faceportion with a polished surface finish exhibiting a sufficiently lowcoefficient of friction to facilitate substantially direct contact ofsaid cutting edge with said rock formation.
 10. A method for cutting asubterranean rock formation under subterranean wellbore pressure,comprising:providing a mass of superhard material defining a cuttingface oriented substantially transverse to an intended direction oftravel thereof and a cutting edge at a periphery of said cutting face;engaging said subterranean rock formation with said cutting edge andmoving said cutting face in said direction of travel; cutting a chip ofmaterial from an uncut portion of said subterranean rock formation withsaid cutting edge, said chip passing over at least a portion of saidcutting face in contact therewith; and preventing substantial adherenceof said chip to said cutting face portion by providing said cutting faceportion with a polished surface finish exhibiting a sufficiently lowcoefficient of friction to avoid a substantial build-up of material cutfrom said subterranean rock formation ahead of said cutting face and tofacilitate substantially direct contact of said cutting edge with saidrock formation.
 11. A drill bit for cutting a subterranean rockformation, comprising:a rotary drill bit body having a connector at oneend thereof for connection of said apparatus to a drill stringcomponent; and at least one cutting element mounted to said bit body andincluding a mass of superhard material defining a cutting face orientedsubstantially transverse to a direction of intended rotation of said bitbody and a cutting edge at a periphery of said cutting face of said atleast one cutting element whereby, upon engagement of said rockformation with said at least one cutting element and rotation of saidbit, a chip of material cut from said formation by said cutting edgepasses over at least a portion of said cutting face in contacttherewith, said cutting face portion having a polished surface finishexhibiting a sufficiently low coefficient of friction to preventsubstantial adherence of said chip thereto, thereby avoiding asubstantial build-up of material cut from said formation ahead of saidcutting face.
 12. A drill for cutting a subterranean rock formation,comprising:a rotary drill bit body having a connector at one end thereoffor connection of said apparatus to a drill string component; and atleast one cutting element mounted to said bit and body and including amass of superhard material defining a cutting face orientedsubstantially transverse to a direction of intended rotation of saidbody and a cutting edge at a periphery of said cutting face of said atleast one cutting element whereby, upon engagement of said rockformation with said at least one cutting edge element and rotation ofsaid bit, a chip of material cut from said formation by said cuttingedge passes over at least a portion of said cutting face in contacttherewith, said cutting face portion having a polished surface finish ofsufficient smoothness to prevent substantial adherence of said chipthereto, thereby avoiding a substantial build-up of material cut fromsaid formation ahead of said cutting face.
 13. A drill bit for cutting asubterranean rock formation, comprising:a rotary drill bit body having aconnector at one end thereof for connection of said apparatus to a drillstring component; and at least one cutting element mounted to said bitbody and including a mass of superhard material defining a cutting faceoriented substantially transverse to a direction of intended rotation ofsaid bit body and a cutting edge at a periphery of said cutting face ofsaid at least one cutting element whereby, upon engagement of said rockformation with said at least one cutting element and rotation of saidbit, a chip of material cut from said formation by said cutting edgepasses over at least a portion of said cutting face in contacttherewith, said cutting face potion having a polished surface finishexhibiting a sufficiently low coefficient of friction to preventsubstantial adherence of said chip thereto, thereby facilitatingsubstantially direct contact of said cutting edge with said uncutportion of said subterranean formation.
 14. A drill bit for cutting asubterranean rock formation, comprising:a rotary drill bit body having aconnector at one end thereof for connection of said apparatus to a drillstring component; and at least one cutting element mounted to said bitbody and including a mass of superhard material defining a cutting faceoriented substantially transverse to a direction of intended rotation ofsaid bit body and a cutting edge at a periphery of said cutting face ofsaid at least one cutting element whereby, upon engagement of said rockformation with said at least one cutting element and rotation of saidbit, a chip of material cut from said formation by said cutting edgepasses over at least a portion of said cutting face in contacttherewith, said cutting face portion having a polished surface finish ofsufficient smoothness to prevent substantial adherence of said chipthereto, thereby facilitating substantially direct contact of saidcutting edge with said uncut portion of said subterranean formation. 15.A drill bit for cutting a subterranean rock formation, comprising:arotary drill bit body having a connector at one end thereof forconnection of said apparatus to a drill string component; and at leastone cutting element mounted to said bit body and including a mass ofsuperhard material defining a cutting face oriented substantiallytransverse to a direction of intended rotation of said bit body and acutting edge at a periphery of said cutting face of said at least onecutting element whereby, upon engagement of said rock formation withsaid at least one cutting element and rotation of said bit, a chip ofmaterial cut from said formation by said cutting edge passes over atleast a portion of said cutting face in contact therewith, said cuttingface portion having a polished surface finish exhibiting a sufficientlylow coefficient of friction to prevent substantial adherence of saidchip thereto, thereby facilitating failure of said formation immediatelyadjacent said cutting edge.
 16. A drill bit for cutting a subterraneanrock formation, comprising:a rotary drill bit body having a connector atone end thereof for connection of said apparatus to a drill stringcomponent; and at least one cutting element mounted to said bit bodyarid including a mass of superhard material defining a cutting faceoriented substantially transverse w a direction of intended rotation ofsaid bit body and a cutting edge at a periphery of said cutting face ofsaid at least one cutting element whereby, upon engagement of said rockformation with said at least one cutting element and rotation of saidbit, a chip of material cut from said formation by said cutting edgepasses over at least a portion of said cutting face in contacttherewith, said cutting face portion having a polished surface finish ofsufficient smoothness to prevent substantially adherence of said chipthereto, thereby facilitating failure of said formation immediatelyadjacent said cutting edge.
 17. A method of modifying the cuttingcharacteristics of a cutting element for use on a drill bit for cuttingrock formations under subterranean wellbore pressure, said cuttingelement possessing a superhard cutting face exhibiting a surfaceroughness in excess of about 20μ in and having a correspondingcoefficient of friction, comprising abrasively smoothing at least aportion of said cutting face to such extent that said coefficient offriction is reduced by at least fifty percent.
 18. A superhard cuttingelement, modified for use on a drill bit for cutting rock formationsunder subterranean wellbore pressure by the method comprising:providinga cutting element possessing a superhard cutting face exhibiting asurface roughness in excess of about 20μ in and having a correspondingcoefficient of friction; and abrasively smoothing at least a portion ofsaid cutting face to such extent that said coefficient of friction isreduced by at least fifty percent.
 19. A method of drilling asubterranean formation, comprising:providing a rotary drag bit with aplurality of cutting elements each having cutting faces, said cuttingfaces having curling edges adjacent curing face portions exhibiting apolished, substantial mirror finish; and engaging said formation withsaid cutting edges under normal force applied to said bit and applyingtangential force to rotate said bit against said formation with saidcutting edges engaged therewith.
 20. A method of drilling a subterraneanformation exhibiting plastic failure properties under wellbore pressure,comprising:providing a rotary drag bit with a plurality of cuttingelements each having cutting faces, said cutting faces having cuttingedges adjacent cutting face portions exhibiting a polished, substantialmirror finish; and engaging said formation with said cutting edges undernormal force applied to said bit and applying tangential force to rotatesaid bit against said formation with said cutting edges engagedtherewith.
 21. A method of drilling a subterranean formation,comprising:providing a rotary drag bit with a plurality of cuttingelements each having cutting faces, said cutting faces having cuttingedges adjacent cutting face portions exhibiting a polished, substantialmirror finish; and engaging said formation with said cutting edges underweight on bit and applying torque to rotate said bit against saidformation with said cutting edges engaged therewith.
 22. A method ofdrilling a subterranean formation exhibiting plastic failure propertiesunder wellbore pressure, comprising:providing a rotary drag bit with aplurality of cutting elements each having cutting faces, said cuttingfaces having cutting edges adjacent cutting face portions exhibiting apolished substantial mirror finish; and engaging said formation withsaid cutting edges under weight on bit and applying torque to rotatesaid bit against said formation with said cutting edges engagedtherewith.
 23. A rotary bit for drilling a subterranean formationexhibiting plastic failure properties under wellbore pressure,comprising:a bit body having a connector at one end thereof forconnection of said bit to a drill string component; at least onesuperhard cutting element mounted to the bit body, oriented to projecttherefrom and having a cutting face bounded at a periphery by a cuttingedge for engaging the subterranean formation, at least a potion of thecutting face adjacent the cutting edge exhibiting a surface sufficientlysmooth to facilitate substantially unimpeded movement thereacross of achip of material cut from said formation under differential pressureconditions otherwise tending to adhere said chip to said cutting faceportion and impede said movement; and a chip breaker for contacting saidchip and facilitating removal thereof from said bit associated with theat least one superhard cutting element and spaced from the cutting edgeso that at least part of the sufficiently smooth surface is interposedbetween the cutting edge and the chip breaker.
 24. A rotary bit fordrilling a subterranean formation exhibiting plastic failure propertiesunder wellbore pressure, comprising:a bit body having a connector at oneend thereof for connection of said bit to a drill string component; atleast one superhard cutting element mounted to the bit body, oriented toproject therefrom and having a cutting face bounded at a periphery by acutting edge for engaging the subterranean formation, at least a potionof the cutting face adjacent the cutting edge exhibiting a surfacesufficiently smooth to avoid substantial build-up of material cut fromsaid formation ahead of said cutting face so that a chip of material cutfrom said formation may move relatively freely across said cutting faceportion; and a chip breaker for contacting said chip and facilitatingremoval thereof from said bit associated with the at least one superhardcutting element and spaced from the cutting edge with at least part ofthe sufficiently smooth surface interposed between the cutting edge andthe chip breaker.
 25. A rotary bit for drilling subterranean formationsexhibiting plastic failure properties under wellbore pressure,comprising:a bit body having a connector at one end thereof forconnection of said bit to a drill string component; at least onesuperhard cutting element mounted to project downwardly from the bitbody and having a cutting face bounded at a lower periphery by a cuttingedge, at least a potion of the cutting face adjacent the cutting edgeexhibiting a surface sufficiently smooth to facilitate substantiallyunimpeded movement thereacross of a chip of material cut from saidformation under differential pressure conditions otherwise tending toadhere said chip to said cutting face portion and impede said movement;and a chip breaker for contacting and bending said chip at an angle toits direction of movement associated with the at least one superhardcutting element and located above at least part of the sufficientlysmooth surface.
 26. A rotary bit for drilling subterranean formationsexhibiting plastic failure properties under wellbore pressure,comprising:a bit body having a connector at one end thereof forconnection of said bit to a drill string component; at least onesuperhard cutting element mounted to project downwardly from the bitbody and having a cutting face bounded at a lower periphery by a cuttingedge, at least a portion of the cutting face adjacent the cutting edgeexhibiting a surface sufficiently smooth to avoid substantial build-upof material cut from said formation ahead of said cutting face so that achip of material cut from said formation may move relatively freelyacross said cutting face portion; and a chip breaker for contacting andbending said chip at an angle to its direction of movement associatedwith the at least one superhard cutting element and located above atleast part of the sufficiently smooth surface.
 27. A method of reducingat least one of normal and tangential forces required to be applied fora given depth of cut in drilling a subterranean formation under wellborepressure with a rotary drag bit employing cutting elements havingcutting faces including superhard material, comprising:providing saidrotary drag bit with a plurality of cutting elements having cuttingfaces including superhard material, said cutting faces having cuttingedges adjacent cutting face portions exhibiting a polished finishproviding a correspondingly low coefficient of friction; and engagingsaid formation with said cutting edges under normal force applied tosaid bit and applying tangential force to rotate said bit against saidformation with said cutting edges engaged therewith; whereby saidpolished, low coefficient of friction finish on said cutting faceportions precludes a substantial build-up of material ahead of saidcutting faces of material cut from said formation, allowing engagementof uncut portions of said formation by said cutting edges and promotingfailure of formation material immediately adjacent thereto.
 28. A methodof reducing at least one of weight on bit and torque required for agiven depth of cut in drilling a subterranean formation under wellborepressure with a rotary drag bit employing cutting elements havingcutting faces including superhard material, comprising:providing saidrotary drag bit with a plurality of cutting elements having cuttingfaces including superhard material, said cutting faces having cuttingedges adjacent cutting face portions exhibiting a polished finishproviding a correspondingly low coefficient of friction; and engagingsaid formation with said cutting edges under weight on bit and applyingtorque to rotate said bit against said formation with said cutting edgesengaged therewith; whereby said polished, low coefficient of frictionfinish on said cutting face portions precludes a substantial build-up ofmaterial cut from said formation ahead of said cutting faces, allowingengagement of uncut portions of said formation with said cutting edgesand promoting failure of formation material immediately adjacentthereto.
 29. A method of increasing the rate of penetration for a givenweight on bit and torque applied in drilling a subterranean formationunder wellbore pressure with a rotary drag bit employing cuttingelements having cutting faces including superhard material,comprising:providing said rotary drag bit with a plurality of curingelements having cutting faces including superhard material, said cuttingfaces having cutting edges adjacent cutting face portions exhibiting apolished finish providing a correspondingly low coefficient of friction;and engaging said formation with said cutting edges under normal forceapplied to said bit and applying tangential force to rotate said bitagainst said formation with said cutting edges engaged therewith;whereby said polished, low coefficient of friction finish on saidcutting face portions precludes a substantial build-up of material aheadof said cutting faces of material cut from said formation, allowingengagement of uncut portions of said formation by said cutting edges andpromoting failure of formation material immediately adjacent thereto.